syntax: make match arms store the expr directly.

Previously `ast::Arm` was always storing a single `ast::Expr` wrapped in an
`ast::Block` (for historical reasons, AIUI), so we might as just store
that expr directly.

Closes #3085.

src/libsyntax/ext/build.rs

@@ -679,7 +679,7 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
         ast::Arm {
             pats: pats,
             guard: None,
-            body: self.block_expr(expr)
+            body: expr
         }
     }